The present invention relates to fluid pressure devices, and more particularly, to a means for limiting the axial movement of the drive shaft relative to the orbital gear set.
In general, fluid pressure devices of the type to which the present invention is adaptable include an internally-toothed ring member, an externally-toothed star member disposed therein for orbital and rotational movement, and a drive shaft in splined engagement with the star member and with either an output shaft (in the case of a hydraulic motor) or an input shaft (in the case of a hydraulic steering unit). The present invention is useful for any type of device which includes an orbital gear set and a drive shaft splined to the externally-toothed star member. It is well suited for use with hydraulic power steering units, and will be described in connection therewith.
In conventional hydraulic steering units, the orbital (or gerotor) gear set serves as fluid meter and is interconnected with the valve means by the drive shaft in such a manner that the drive shaft is axially movable relative to the externally-toothed star member.
The desirability of limiting axial movement of the drive shaft relative to the star member has long been recognized to insure both that the wear pattern between the male splines of the drive shaft and the female splines of the star member is stabilized and that the torque transmitted from the drive shaft to the star member is distributed over the full length of the mating male and female splines. It is also necessary to prevent the drive shaft from moving axially far enough to become disengaged from the drive pin linking the shaft to the spool valving.
One approach to limiting axial movement of the drive shaft has been to insert a spacer member within the central bore defined by the externally-toothed star member. The spacer member would engage the end of the drive shaft and extend to the end of the bore, contacting the inner surface of the cover or end cap bolted on the rearward end of the gerotor set.
The use of such spacers not only increases the cost and complexity of the manufacturing and assembly procedures, but also, any rotational and axial movement of the spacer member may increase the operating noise level of the hydraulic device. A further disadvantage of such spacers is that they effectively increase the mass of the star, thus increasing the kinetic energy of the star during its orbital motion and the resulting wear and vibration.
Accordingly, it is an object of the present invention to provide a fluid pressure device including a means for limiting axial movement of the drive shaft which requires no additional parts which serve no other function and no parts which are relatively free to move, thus causing noise.
Another approach is illustrated in U.S. Pat. No. 3,549,283, and the improvement thereon illustrated in U.S. Pat. No. 3,657,903. Both of the cited patents involve the use of a shoulder on the drive shaft which engages a wall within an annular groove defined in a "valve plate". As was noted in the second of the cited patents, its purpose was to minimize the amount of rotational and orbital rubbing between the shoulder and the "stop surface". However, even the improvement in the design permits a substantial amount of rubbing action between the rotating and orbiting shoulder portion on the drive shaft and the fixed stop surface in the annular groove. Also, the valve plate illustrated in the cited patents, could, in many cases, be eliminated if it is not needed to provide a stop surface.
Accordingly, it is another object of the present invention to provide a means for limiting axial movement of the drive shaft which substantially eliminates both rotational and orbital rubbing action between engaging surfaces, and the resulting wear of the rubbing surfaces.
It is a more specific object of the present invention to provide such a means which utilizes engagement of surfaces rotating and orbiting at substantially the same speed.